Building linear amplifier prototypes

We know way too little about this subject but hopefully [Bob4analog] helped us learn a little bit more this time around. He’s building his own linear amplifiers on what looks like sheets of MDF. This is an evolving design and the two videos after the break show two different iterations. He’s salvaged several components, like transformers from microwaves, as well as built his own components like the plate choke to the right of the tubes in the image above. In standby, the amp sits at 2800 volts, warming the filament before the unit is switched on.

So what’s he got planned for this? Good question, but it appears that there’s more than enough power to drive a long-range transmitter.

Tube amps are really pretty simple devices, note that the 2800 Volts is not used to warm the filaments, that is done by the six volts from the rewired microwave transformer.
The current throught the tube is modulated by the input voltage, and is in the range of about 100..200 milliamps (at 2800 Volts). The tube is in series with a output transformer which transforms the high voltage/low current signal to a more suitable voltage/current combination.

I think a lot electronic enthusiasts outside of the radio world fail to understand that the vacuum tube still has an important role in amplification of RF (radio frequency) at very high power levels for radio transmitters. In this case the creator of this video decided to “roll his own” instead of pay $ 1000.00 or more for a commercial unit. What is commendable is that he rewound inexpensive MOT (Microwave Oven Transformers)for the power supply – most transformers designed for RF amplifier use are very expensive to purchase. And this HAM radio operator used ingenuity. Please be aware that MOTS are half wave transformers with very little current limitation, the shock they provide is not nasty, but VERY LETHAL.

@ Fred Cousins, I semi agree with you on your statement about the vacuum tube, however high power solid state amplifiers are more common these days than vacuum tubes.

Don’t get me wrong, there are still vacuum tubes around, however not many/any companies will buy vacuum tube radio transmitters as they are too difficult to replace parts on, and too difficult to maintain.

Still very much enjoy the hack, always love to see amateur radio hacks!!

Frist let me say “yes linears are simple rf amplifiers” and that’s has simple as simple can go as far as what’s really happening. First the guy that said the filments is only running 6 volts usually is 5 to 5.5 but current wise it’s 20 amp and above. Now the plate can be anywhere between 1200vdc at 1 amp+ to 4500vdc at 1 tp 2.5 amps and alot of that depends on the match with the antenna system, by the way a bad mismatch at this point can cost alot in tubes like $550.00 ea with a bad mismatch and rf current making a u turn at at he antenna taking out the rf deck also blowing the final matching tuner and we havn’t even staarted talking about the possibility of fire to you and your little brother and family and yoir radio itself as well as a house fire. So please don’t take them for granted and this guys seem professional enough and it has a good lay out also. so 73 from Gregory Ellis goodmorning an goodnight from KJ4AYH PS Please becarful with these toys!!!!

The 572B’s are like under 50eur if you know where to look and russians sell those on ebay for 80bucks too. And tubes are quite rugged, one can actually manage to avoid disaster by reacting quickly enough. None of the surprise cratering like with transistors.

hope I didn’t make anybody mad just telling it like I read it and the amps I;ve built myself…his just seemed a little, better organized that mine was, which was a good changed. Please give me some feed back on the use of the MOT for transformer replacement for the more expensive one that is called for…and I mean alot more expensive like $525.00+ When I can usually take a 1100-1300 watt industrial or resturant microwave buy it used for $30.00 to$50.OO dollars if the problem wasn;t the transformer in the first place and I;ve got my transformer and high current dioide also..by for now from gregory KJK4AYH

Wow, I just happened upon this site…I had no idea my creation had such a liking. I’m very honored. Just to be sure I mention it, the linear amp I made in this vid is mostly for demonstrating how one could salvage parts from something like a microwave oven and make something like this. Dangerous? Absolutely! (Of course, driving a race car at 230 MPH is dangerous too.) But nobody is going to operate such a device in this fashion in everyday use. Extreme caution is always a must when experimenting with high voltage. Anyway, I had fun making it and I’m glad it’s of interest to others. I’m not the only one doing this either. See Ed’s breadboard amp… https://youtu.be/ywQrfZeWIg0